Cathode-ray beam deflecting circuit



April 12, 1949.

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CATHODE-RAY BEAM DEFLECTING CIRCUIT Filed Jan. 13, 1945 TMA/@FORMER SECOND/IRF Ul F550 wick Feo/vf mb a4 0^/ JEVC/VAEV CURRENT DUE TU TUBE 10 FWZ.

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/1 TTORN'Y Patented Apr. 12, 1949 UNITED STATES PATENT OFFICE CATHODE -RAY BEAM DEFLECTING CIRCUIT Otto II. Schade, West Caldwell, N. J., assigner to Radio Corporation of America, a corporation of Delaware Application January 13, 1945, Serial No. 572,712

is of such shape as to cause the movement of the cathode ray beam to be at a constant rate with respect-to time during the useful part of each deection cycle.

It is a customary practice in circuits of the above type to provide an electron discharge tube having a relatively high power output for supplying energy to the beam deflecting coils. Across the deflecting coils a so-called damping tube is frequently connected. This tube is conventionally represented as comprising an inverted diode,

the purpose of which is to eliminate any highfrequency oscillations which would otherwise be set up in the deflecting coils, and also to aid in the deflection f the cathode ray beam during a part of each deflection cycle.

The inverted diode serves as a periodic discharge path for the electromagnetic energy stored in the deiiecting coils vor other circuit inductance at the end of the deflection cycle. In the absence of the diode, this stored energy would `normally produce relatively high-frequency oscillations in the systems, with the result that the deflection of the cathode ray beam during the initial part of its deflection cycle would be non-linear. When the diodeis used as a damping tube, only one-half cycle of free oscillation is permitted to take place, and thereafter the energy contained in the defiecting coils or other circuit inductance is used for the initial part of the next useful deflection cycle.

When a conventional power tube and an inverted diode are used as above described, it is found that the summation characteristic of these tubes is not linear, and as a consequence distortion of thev reproduced image may occur. To overcome this condition, an arrangement has been proposed in which feedback voltages are applied to the control electrode of the power tube. A circuit employing the use of feedback voltage variations is illustrated and described in Schade application Patent No. 2,309,672, granted February 2, 1943.

Ithas also been found that linearity of deflection may be :obtained by modifying the .characteristics of Athe .inverted diode .or `suppressor tube. 'Ioy permit such modification, the diode is replaced bya triode or multi-grid electron disl charge tube, so that-voltage variations of a particular Wave form may be applied to the control electrode of the discharge tube to alter its conduction characteristics. A system of this nature is disclosed in United States Patent No. 2,382,822 granted to Otto H. Schade. When the conduc- ,tion characteristics of the suppressor tube are varied during each deflection cycle, linearity of deflection may be produced without resorting to the use of feedback voltage variations applied to the control electrode of the power tube.

A scanning circuit such as shown in abovementioned Patent No. 2,382,822, in which a controlled suppressor or damping tube is utilized with transformer coupling between power tube and deflection coils, has a number of advantages over the sO-called direct-drive systems when the coupling transformer is designed with a stepdown ratio. Among these advantages is the fact that the capacitances of yoke, cable, and damping tube are removed from the primary and placed in the secondary circuit, permitting the building up of a higher plate circuit inductance to thereby result in a gain in yoke current. Also, the surge voltage on the damping tube, yoke, and

cable is reduced in value so that the insulation requirements are lessened.

-, However, operation of a transformer-coupled circuit such as shown in Patent No. 2,382,822 normally requires that the polarity of the current in the secondary circuit be the same as that in the primary or power tube circuit. For that reason the damping tube, which is yconductive alternately with the power tube and with power supplied by the primary circuit, must be actually inverted or, in other words, positioned so that its cathode is connected to the high potential end of the deecting coils. From a practical stand-y point, a circuit of this type not only requires an isolation transformer for the tube heater, but in addition necessitates employing a damping tube designed especially for such a mode of operation. Furthermore, the voltage of the control electrode inthe damping tube is floating at surge potential, and is difficult to measure or adjust.

According to one feature of the present invention, I provide means whereby the damping or suppressor tube may be operated with its cathode at A.C. ground potential, thereby permitting the tube heater to be grounded and eliminating the necessity for an isolation transformer or other special circuit components.

As above brought out, the power tube builds up magnetic energy in the deection coils or other circuit inductance in the latter part of the Scanning period. During retrace, the polarity of the magnetic iield, and consequently the current ow in the circuit inductance, is reversed in onehalf cycle of free oscillation. The reversed current is now forced to decrease at the proper rate over the controlled suppressor or damping tube, and this reversed current supplements the power tube current to a linear summatn characteristic. In order to obtain linearity of this summation characteristic, the wave form of the control voltage on the grid of the suppressor tube must be such as to eect a proper control of the rate of current decrease through the damping tube.

In Patent No. 2,382,822, above mentioned,` the control grid voltage for the suppressor tube is obtained by diiierentiation of the surge voltage across the defiecting coils. The result of this diiierentiation is a falling sawtooth Voltage corresponding with reversed polarity to the voltage applied to the grid of the power tube.

According to a further feature of the present invention, I provide means for deriving for` the gridof the suppressor tube a control voltage which has two exponential sections, and is of such form that parasitic oscillations in the ycircuit inductance appear in proper phase in the grid circuit of the suppressor tube to produce degeneration of such oscillations in the anode circuit. The control grid voltage thus derived is especially eiective in damping out the first strong parasitic oscillations atthe beginning of scansion.

A complete current cut-oi in the plate circuit of the power tube during retrace periods is essential for high eiiiciency. This is very noticeable in circuits having a low shunt loss, such as that herein disclosed, where the plate impedance of a partially cut-E tube represents a large portion of the shunt. Often, however, the syncho nizing pulse generated by means such as a blocking oscillator is too short forlcomplete power tube cut-ofi.

According to a still further feature of the present invention, a negative peaking pulse off proper duration is derived from the secondary circuit and fed back to the control electrode of the power tube. It has been found that no4 adjustment of driving voltage, wave shape, bias, or blocking oscillator peaking in the circuit gives the eiiiciency obtained with a negative peaking pulse of proper duration, especially when it is appreciated that a square current cut--oii` such as that obtained by the means herein disclosed reduces to a considerable. extent the plate dissipation in the power tube.

It is, therefore, one object of the present in. vention to provide a circuit for electromagneti-` cally deiiecting a cathode ray beam in which linearity of deflection may be insured through. an improved control circuit for the damping or suppressor tube.

Another object of the present invention resides in the provision of a controlled suppressor tube in a circuit arrangement for electromagnetically deflecting a cathode ray beam, and so controlling the suppressor tube by means of a voltagehaving a wave form of exponential shape that the summation of thecurrent flow through the suppresr, sor tube and through the` deflection power` tube during each cycle will produce substantially linear deection of the cathode ray beam.

A still further object ofthe present invention resides in the provision of a system for electro-` magnetically deflecting a cathode raybeam in which a controlled suppressor or damping tube f forni` of' the invention and from the drawing, in

which:

Fig. 1 shows schematically a cathode ray beam deecting circuit in accordance with the present invention :1` and Fig. 2 is a set of waveforms illustrating the operation of the circuit of Fig. 1.

Referring nowI to Fig. 1, there is shown a deleotionl power tube I0- whioh supplies current to p a pair of deectingcoils I2 through a transformer |24'. Tube Hl ispreferably` of'a type such asrthe` RCA 807 and includes a cathode, a control electrode, a screen electrode, and an anode. The anode ofl tube Hl` is connected to.. they positive` terminal I6 of a suitable source of potential (not shown) through the primary winding of trans former i4., soV that the latter represents the load circuitofthetube. Between the cathode of tube= lrfand ground is connected a bias resistor l8; which is shuntedbyaby-pass condenser 20. The screen` electrode of tube l0 is maintained at,` propervpotential with respect to, theacathode by' means of .two seriesresistors 22; and 2411connected: beinveen` the screen electrode and` theV positiveA terminal |18 of the potential source. Resistor 2,4. is adjustable. provided between the screen electrode and ground.

For deriving acontrol voltage for the control electrode.- oi tube t0, there is;` provided a condenser 28; which. is charged from thesource of potential connected to the. terminal- |16, throughresistors 22` and 24 and; an additional charging resistor 3,0. The voltage` developed on condenser 28 is: applied to. the control electrode of; tube Nl.y through: a coupling condenser 3:2; Aresistor 34, ofrelativelr high value is connectedbetween the cont-roi elec# trode of; tube lallxld round. TheA chargingrate of4 condenser 2% may be varied by a.f1.ii1strr1ent-` 0f resistor 2A, as will, be later described.

It is desired that the voltage, appearing: on the control electrode of tube, IMD be of` ashapesuch, as,

representedL by` Weile form 36 or,` in other words,

ofrising; sawtootli configuration. In order toobtamf a, our-.ve of this naturel, there is. provided means for discharging thet condenser` 28, this means. in theA form, shown comprising a.` grid-con.. trailedA discharge tube 3i!` of any suitable type, with thedischaree tube anodeheins connected t0, the` upperplat@.if` Condenser 28- Discharge tube. 3,8 is normally biased to cut-oft by some conventional: means suchas a b iasl resistor 4Q connected between the grid or the tube and the negative terminal cfa source or bias potential (not shown) When tube 38 is non-conductive condenser 2B will be charged through resistors-22, 24, and30 to produce a chargeon condenser'such asrepresentedby the rising positive portion of saw-` tooth wave 35. By selecting elements having a, suitable time constant, thisrisingpositivefportion of` wave 36 may be made exponential; or substantially linear, its: amplitude being4 con A screeny lay-pass, condenser 2t onthe control electrodes as represented in curve E is changed exponentially in a negative direction. This reduces at a predetermined rate the current ow through the suppressor tube 50, thereby resulting in a summation current that is linear and that will produce linear deection of the cathode ray beam.

The means for obtaining a voltage variation having a wave form such as represented by the curve 64 includes a differentiating network comprising a condenser 66 and resistors 68 and 14, the upper plate of condenser 66 being connected to the upper end of the secondary winding of transformer I4. Bias for the control electrodes of suppressor tube 50 is obtained by grid voltage vrectiilcationand grid current in resistor 10 in parallel with a by-pass condenser 12, one endv ofA resistor 10 being connected to the joined cathodes of suppressor tube 50. Between resistors 68 and 10 is an adjustable resistance element comprising a potentiometer 14, the latter acting as a linearity control.

The voltage output of the differentiating network as developed on resistors 68` and 14 is applied to one control electrode of suppressor tube 50 through a shaping resistor 16, and to the other control electrode of suppressor tube 50 through a further shaping resistor 18. Resistors 16 and 18 are of equal value so that the voltage appearing on each of the control electrodes of suppressor tube 50 will have the same wave form.

The voltage appearing on the control electrodes of suppressor tube 50, as represented by the wave form 64, has two exponential sections respectively occurring during intervals t1 and t2 as indicated in Fig. 2E. When the control electrodes of suppressor tube 50` are positive, current iiows through resistors 1B and 18 and the grid-cathode impedance of tube 50. This total impedance `shunts the differentiating resistor 68 so as to change the differentiating constant of the differentiating network 66, 68. The result is a fastdecaying transient during interval t1, and this voltage variation produces a heavy initial plate current output from suppressor tube 50 which is eiective in damping out the first strong parasitic oscillations in the circuit.

The second section of wave form 64 is a slowdecaying voltage during interval t2 caused by the resistor 68 when the control electrodes of suppressor tube 50 are negative and no current flows through resistors 'I6 and 78. These resistors 16 and 18, therefore, act in effect as current limiters to vary the impedance of the control electrodes of suppressor tube 50.

The result of the above mode of operation is to improve the linearity of the system by regulating the current output of the suppressor tube by a control voltage of such form as to cause the current now through the suppressor tube to decrease in a predetermined manner.

As set forth above and as shown in Figs. 1 and 2, the wave form `38 of the voltage appearing on the control electrode of power tube I has high negative peaks. These peaks are obtained by means of a peaking resistor 4B having a sliding contact 44 connected to the lower plate of charging condenser 28. One end of peaking resistor 46 is connected to a tap 84 on the secondary winding of transformer I4, permitting a negative peaking pulse of proper duration to be fed back from the transformer I4 to the control electrode of power tube I0. The amplitude of the peaking pulse may be varied by adjustment of contact 44.

It has been found in practice that the following values are suitable for the circuit components usedin the system herein described. However, these values are being given merely as an example, and it is to be understood that other values may be substituted for any or all of the components if desirable or necessary.

Value should be such that the screen dissipation of the power tube I0 will not be exceeded.

Ohms 24 f 10,000 68 50,000 74 250,000 '.76, 78 5,000 70 100,000 52 500 58 5 to 10 While I have shown and described, and have pointed out in the annexed claims, certain novel features of my invention, it will be understood thatlvarious omissions, substitutions, and changes in the form and details of the system illustrated maybe made by those skilled in the art without departing from the spirit of the invention.

I claim:

1. A deecting system for a cathode ray beam comprising a deflection power tube having cathode, con-trol and anode electrodes; a beam deflectioncoil, a transformer having its primary winding in the anode-cathode circuit of said power tube and its secondary winding connected across said deection coil, means for applying voltage variations of predetermined waveform between the control electrode and cathode of said power tube to supply said deiiection coil with an approximately sawtooth current through the transformer, said transformer being wound so that the corresponding ends of its primary and secondary windings have opposite relative polarities, a lamping tube having cathode, control and anode electrodes; means connecting the anodecathode electron path of said damping tube across said deflection coil, the anode of said dampingtube being connected to that end of said coil more remote from ground, means for differentiating the voltage developed across said deflection coil and applying said differentiated voltage to the control electrode of said damping tube, and means including the control electrode-cathode impedance of said damping tube for modifying the wave shape of said differentiated voltage to provide a damping tube current which supplements the power tube current whereby their algebraic sum in the deflection coil is substantially linear. i

2. A delecting system according to claim 1 wherein said last mentioned means comprise a resistor connected in series between the control electrode of said damping tube and said differentiating means.

3. In a beam deflecting system of the type having a transformer secondary, a damping circuit, and a deection coil all connected in parallel, the combination of a damping tube having cath ode and anode electrodes in said damping circuit, a control electrode in said damping tube, differentiating means connected between said anode and cathode, and a resistance connecting tie output of said diiierentiating means to said control electrode, said resistance having such value relative to the other circuit components to make the damping tube current supplement the current from said transformer secondary, whereby the algebraic sum or" said currents in the deflection coil is substantially linear.

4. A deflecting system according to claim l comprising in addition, means for applying a portion of the energy developed across the secondary winding of said transformer, during retrace, as a negative voltage to the control elec trode of said power tube.

5. In a television circuit, a power tube, a deflection coil, a transformer coupling the output of said power tube to said coil, a damping tube connected across the secondary winding of said transformer and across said coil, said damping tube having a control electrode, means for differentiating the voltage variations induced across said coil, means in series with said control electrode for shaping the differentiated voltage variations to provide a damping tube current which is complementary to the current from said transformer secondary winding, whereby the algebraic sum of said currents in the deflection coil is substantially linear.

6. In a beam deflecting system the combination of, a power tube having an input circuit and an output circuit, a deflection coil, means coupling said output circuit to said deflection coil, a damping circuit including a unilaterally conducting device connected across said coupling means, the arrangement being such that when the output current of said power tube is interrupted the consequent collapse of flux in said deflection coil induces a high potential in said output circuit of a polarity in the direction of conduction of said power tube and in the direction of non-conduction of said unilateral device; a peaking resistor in said input circuit, and means connected between said peaking resistor and said coupling means for feeding back a portion of said induced high potential to increase the blocking effect produced by said peaking resistor on the control electrode of the power tube for the duration of said high potential.

7. In a beam deiiecting system the combination of, a power tube having at least a cathode, a control electrode, and an anode; a transformer having its primary winding connected between said anode and cathode, a deflection coil connected across a secondary winding of said transformer, a damping circuit including a unilaterally conducting device connected across the secondary winding and the coil, means including a peaking resistor connected between said control electrode and cathode to develop voltage variations of sawtooth waveform, means to apply the developed voltage variations to said control electrode, and a feed-back connection from a point l0 cn the secondary winding of said transformer to said peaking resistor to feed back an impulse of negative polarity to said control electrode.

8. The combination of claim 5, in which said voltage variations periodically reverse in polarity, and in which said differentiating means includes a resistance-condenser combination the resistance portion of which is connected in the control electrode-cathode circuit of said damping tube, whereby, when said voltage variations are of positive polarity, current will flow between the cathode and control electrode of said damping tube through said shaping means to effectively shunt the resistance portion of said resistance-condenser combination and thereby alter the differentiating constant of said differentiating means.

9. A circuit for deecting a cathode ray beam during active and retrace intervals comprising the combination of an output tube having at least a cathode, a control electrode and an anode, an input circuit for applying deflection signal p0- tentials to the control electrode-cathode circuit of said output tube, a deflection coil, an output circuit including the anode-cathode elements of the output tube to connect the tube output for energizing the deflection coil, damping means for the deflection coil, a circuit connected to the tube output circuit for deriving impulses during deflection retrace intervals, and for supplying said impulses to the tube input circuit to increase the eective potential between the tube control electrode and cathode thereby to hold the tube at at least cutoff potential during the retrace interval, a condenser included in the input circuit of said output tube, means to couple one electrode of said condenser to the control electrode of said output tube, and means including an impedance to connect the other electrode of said condenser to a point of fixed potential, and wherein the derived impulses are applied to a point on said impedance.

10. A circuit for deilecting a cathode ray beam during active and retrace intervals comprising the combination of an output tube having a cathode, a control electrode, and an anode, a condenser connected in the control electrode-cathode circuit of said tube, a deflection coil, an output circuit including the anode-cathode elements of said output tube connecting the tube output to the deflection coil, damping means for the deflection coil, means for generating a saw-tooth wave of voltage across said condenser, an auxiliary circuit connected to the tube output circuit for deriving impulses during deflection retrace intervals, means connecting said auxiliary circuit in series with said condenser for maintaining the output tube at cutoff during the retrace interval.

OTTO I-I. SCI-IADE.

REFERENCES CETED The following references are of record in the ile of this patent:

UNITED STATES PATENTS Number Name Date 2,280,733 Tolson Apr. 21, 1942 2,308,908 Bahring Jan. 19, 1943 2,370,426 Schade Feb. 27, 1945 2,382,822 Schade Aug. 14,. 1945 

